This invention relates to a method for preparing polyolefins having a bi- or multimodal molecular weight distribution. This invention also relates to a polyolefin polymerization catalyst system. This invention further relates to a method for preparing an olefin polymerization catalyst system.
Polyolefins having a multimodal molecular weight distribution (MWD) can be converted into articles by extrusion molding, thermoforming, rotational molding, etc. and have advantages over typical polyolefins lacking the multimodal MWD. Polyolefins having a multimodal MWD may be processed more easily, i.e. they can be processed at a faster throughput rate with lower energy requirements and at the same time such polymers evidence reduced melt flow perturbations and are preferred due to improved properties for applications such as high strength films.
There are several known methods for producing polyolefins having a multimodal MWD; however, each method has its own disadvantages. Polyolefins having a multimodal MWD can be made by employing two distinct and separate catalysts in the same reactor each producing a polyolefin having a different MWD; however, catalyst feed rate is difficult to control and the polymer particles produced are not uniform in size, thus, segregation of the polymer during storage and transfer can produce non-homogeneous products. A polyolefin having a bimodal MWD can also be made by sequential polymerization in two separate reactors or by blending polymers of different MWD during processing; however, both of these methods increase capital cost.
European Patent No. 0128045 discloses a method of producing polyethylene having a broad molecular weight distribution and/or a multimodal MWD. The polyethylenes are obtained directly from a single polymerization process in the presence of a catalyst system comprising two or more metallocenes each having different propagation and termination rate constants, and aluminoxane.
There are certain limits to the known methods for preparing bimodal molecular weight distribution or multimodal molecular weight distribution polyolefins. Even under ideal conditions the gel permeation chromatograph curves don""t show a marked bimodal MWD of the polyolefin. The MWD and shear rate ratios of the polymer and the catalyst activity disclosed in the known methods are rather poor. Further the known metallocene catalyst systems for producing bimodal MWD use aluminoxane as cocatalyst during the polymerization which causes severe fouling inside the reactor and renders the use of such a type of catalyst in continuous processes almost impossible.
It is therefore not surprising that none of the known methods for producing a multimodal MWD polyolefin from a single polymerization process in the presence of a catalytic system comprising at least two metallocenes have been developed at an industrial scale.
It is an object of the present invention to provide for a new process for preparing polyolefins having a multimodal molecular weight distribution. It is an object of the present invention to provide a new high activity polymerization catalyst system. It is a further object of the present invention to provide for a new process for preparing the polymerization catalyst system of the present invention.
In accordance with the present invention, polyolefins having a multimodal or at least bimodal molecular weight distribution are prepared by contacting in a reaction mixture under polymerization conditions at least one olefin, a catalyst system comprising (a) a supported catalyst-component comprising an alumoxane and at least two metallocenes containing the same transition metal and selected from mono, di, and tri-cyclopentadienyls and substituted cyclopentadienyls, of a transition metal wherein at least one of the metallocenes is bridged and at least one of the metallocenes is unbridged and (b) a cocatalyst.
While alumoxane can be used as a cocatlyst, the Applicant has found that is was not necessary to use alumoxane as cocatalyst during the polymerization procedure for preparing polyolefins according to the process of the present invention. Further the use of alumoxane as a cocatlyst during the polymerization may lead to the fouling of the reactor.
According to a preferred embodiment of the present invention, one or more cocatalysts represented by the formula MRX are used, wherein M is a metal selected from Al, B, Zn, Li and Mg, each R is the same or different and is selected from halides or from alkoxy or alkyl groups having from 1 to 12 carbon atoms and x is from 1 to 3. Especially suitable cocatalysts are trialkylaluminium selected from trimethylaluminium, triethylaluminium, triisobutylaluminium, tri-n-hexylaluminium or tri-n-octylaluminium, the most preferred being triisobutylaluminium.
In accordance with the present invention the broadness of the molecular weight distribution and the average molecular weights can be controlled by selecting the catalyst system. In a preferred embodiment of the present invention, this control is also preferred by the introduction of some amount of hydrogen during polymerization. Another preferred embodiment of the present invention implies the use of a comonomer for this control; examples of comonomer which can be used include 1-olefins such as 1-butene, 1-hexane, 1-octene, 4-methyl-pentene, and the like, the most preferred being 1-hexene.
It has unexpectedly been found that the polymerization process can be conducted under slurry phase polymerization conditions and this constitutes a real advantage of the process of the present invention. While slurry phase polymerization may be conducted under well known operating conditions, it is preferred that it is operated at a temperature of about 20 to 125xc2x0 C. and a pressure of about 0.1 to 5.6 MPa for a time between 10 minutes and 4 hours.
Another advantage of the present invention is that a continuous reactor can be used for conducting the polymerization. This continuous reactor is preferably a loop reactor. During the polymerization process, the olefin monomer(s), the catalytic system, the cocatalyst and a diluent are flowed in admixture through the reactor.
A further advantage of the present invention is that the bulk density of the polymer obtained by the process of the present invention is particularly high. The bulk density is an important characteristic of the polymer. The bulk density, commonly expressed in terms of grams per cubic centimeters, should be relatively high. If the bulk density is too low, the polymer will tend to be fluffy and will tend to cause plugging and handling problems in the product transfer system. Low bulk densities mean problems for fluff packaging and for the extrusion processing. This is particularly important in a continuous or a semi-continuous polymerization where plugging of the withdrawal outlet or another point in the polymerization system can cause serious interruptions in production schedules.
According to the present invention when hydrogen is used it is preferred that the relative amounts of hydrogen and olefin introduced into the polymerization reactor be within the range of about 0.001 to 15 mole percent hydrogen and 99.999 to 85 mole percent olefin based on total hydrogen and olefins present, preferably about 0.2 to 3 mole percent hydrogen and 99.8 to 97 mole percent olefin.
It is preferred that the polymerization reaction be run in a diluent at a temperature at which the polymer remains as a suspended solid in the diluent. Diluents include, for examples, isobutane, n-hexane, n-heptane, methylcyclohexane, n-pentane, n-butane, n-decane, cyclohexane and the like. The preferred diluent is isobutane.
The olefin monomer used in the process of the present invention to produce a polyolefin of bimodal of multimodal molecular weight distribution in which each polymer particle contains both high and low molecular weight polymer molecules is preferably selected from ethylene and mono-1-olefins (alpha olefins), preferably mono-1-olefins having from 2 to 10 carbon atoms including for example, 4-methyl-1-pentene. More preferably these mono-1-olefins are selected from the group consisting of ethylene, propylene, and mixtures thereof; ethylene being the most preferred.